Implantable cardiac pacemakers have been developed to detect undesirably rapid contractions of the heart which are characterized as tachyarrhythmias or tachycardias. It is known that such pacemakers may terminate or interrupt a detected tachycardia by applying stimulation pulses to the heart which coincide with a termination time period for the tachycardia. Such stimulation pulses may be applied in bursts (one or more pulses). In operation, the time at which a burst begins, the number of pulses in the burst and the time interval between such pulses may be programmed to a fixed value or may be adaptively determined as a percentage of the detected rate of the tachycardia. It has also been proposed that the tachycardia treatment pulses may have intervals which automatically decrement within the burst. Alternatively, the time at which the burst begins or the intervals between pulses of the burst may be "scanned" by incrementing and/or decrementing these values by programmed amounts as bursts are applied to terminate a tachycardia.
Typically, antitachycardia pacemakers attempt to terminate tachycardias when the heart rate exceeds a predefined high rate. Although this means for detecting a tachycardia has the advantage of simplicity, it has the disadvantage that it will detect and attempt to treat high rate sinus rhythms which result from exercise and which the pacemaker should not attempt to terminate. Also, use of the simple high rate detection criterion will result in an attempt to treat other non-pace-terminable conditions.
It is therefore desirable to develop detection criteria which can distinguish and treat only those tachycardias which are susceptible to pacer-termination, such as reentrant tachycardias. It has been suggested that a pace-terminable tachycardia may be more accurately identified by detecting the rate of onset of the tachycardia. Suddenness of onset often indicates a reentrant tachycardia which may be treated by the pacemaker. High rate sinus rhythms resulting from exercise will not be detected as pacer-terminable tachycardias, because they do not have the characteristic of sudden onset.
Although the sudden onset detection criterion is useful in distinguishing pacer-terminable tachycardias, it is not capable of discriminating between all such tachycardias. Moreover, a simple sudden onset criterion could mistakenly indicate a pacer-terminable tachycardia as a result of transitory cardiac conditions, such as the compensatory pause that normally follows a premature cardiac contraction occurring during a period of high rate cardiac activity.
It is therefore desirable to provide an improved system for reliably detecting pace-terminable tachycardias. Accordingly, it is an object of the invention to provide a pacemaker which reliably detects pace-terminable tachycardias in accordance with programmed detection criteria which are defined with respect to the needs of a patient.
Another object of the invention is to provide a pacemaker which employs the criteria of high rate, sudden onset, rate stability and sustained high rate to distinguish pace-terminable tachycardia conditions which may be treated.
A further object of the invention is to provide such a pacemaker wherein the criteria may be selectively combined to maximize the probability of detecting pace-terminable tachycardias for a particular patient.
Another object of the invention is to provide a pacemaker with an improved sudden onset tachycardia detection criterion which distinguishes actual pace-terminable tachycardias from transitory cardiac conditions which may-occur as a result of a compensatory pause following a premature cardiac contraction during a period of high rate cardiac activity.
A further object of the invention is to provide a pacemaker with a rate stability criterion which compares the present high cardiac rate with an average cardiac rate determined from prior rate measurements and diagnoses a pace-terminable tachycardia if the present rate is within a predefined range of the average rate.
Another object of the invention is to provide pacemaker with a sustained high rate criterion which will cause a tachycardia to be treated if a predefined high cardiac rate is sustained for a predetermined period, even if other selected cardiac detection criteria have not been met.
It has been suggested that an antitachycardia pacemaker may treat a tachycardia with treatment parameters which were successfully applied to treat a previous tachycardia. Although this general procedure has the advantage, on the average, of reducing the time required to treat successive tachycardias, it has the disadvantage that it may extend the time for treatment by attempting to treat a tachycardia of one rate with treatment modalities which-were previously successful in terminating a tachycardia having a very different rate.
It is therefore an object of the invention to provide an antitachycardia pacemaker which remembers the values of the parameters of treatment modalities which were successfully applied to one tachycardia and which applies these remembered values to treat a similar tachycardia.
A further object of the invention is to provide an antitachycardia pacemaker which conditions its use of remembered values of treatment modalities on a comparison of the present tachycardia rate with the tachycardia rate of the previous successfully treated tachycardia.
Another object of the invention is to provide an antitachycardia pacemaker which utilizes primary and secondary methods for treating a tachycardia.
It is a further object of the invention to provide a pacemaker wherein the secondary treatment method may be applied first, if the treatment history indicates that this method has been more successful than the primary method for treating a similar tachycardia.
Another object of the invention is to provide a pacemaker wherein a detected pace-terminable tachycardia is treated by preselected primary and secondary methods and, if these methods do not terminate the tachycardia, the pacemaker optionally restarts the treatment methods to again attempt to terminate the tachycardia.
These and other objects of the invention will become apparent from a review of the drawings and the detailed description of a preferred embodiment which follows.